It is well known to prepare olefin polymer foams by heat plastifying a normally solid olefin polymer resin, admixing such heat plastified resin with a volatile blowing agent under heat and pressure to form a flowable gel and thereafter extruding the gel into a zone of lower pressure and temperature to expand and cool the gel to form the desired solid olefin foam product.
A problem frequently encountered is that of preventing an unacceptable degree of shrinkage of partially cured foam during the aging or curing period following manufacture. During the "aging" or "curing" period, the blowing agent employed is gradually diffusing out of the cells in the foam product and air is gradually diffusing into the cells in place thereof. For example, U.S. Pat. No. 4,347,329 teaches that a blowing agent such as dichlorodifluoromethane can be used, and that it can be mixed with other stability agents and volatile halogenated hydrocarbons to produce low density (e.g., 1 to 6 pounds per cubic foot) foams of ethylenic polymer resins.
Other existing technology has also developed along these same lines to allow preparation of dimensionally stable foams (e.g., U.S. Pat. No. 3,644,230 and Watanabe et al., U.S. Pat. No. 4,214,054). These techniques, however, limit the maximum cross-section of the foam articles which can be obtained from a given type of extrusion apparatus. Moreover, changes of volatile blowing agents, and mixtures thereof with and without stability control modifiers affect the foam production in a variety of undeterminable ways.
The blowing agents used in the production of these low density foams are generally a high percentage, if not entirely, fluorocarbons. Saturated chlorofluorocarbons are suspected of destroying the earth's protective ozone layer by migrating up through the troposphere to the stratosphere and catalyzing ozone-destroying chain reactions. Because these compounds are so stable, they are able to survive in the troposphere, the troposphere being the layer of the atmosphere extending from Earth's surface to an altitude of 10 to 16 miles. Since the 1950's, rapid advancement in chemical analytical techniques have led to a large amount of knowledge concerning atmospheric composition. It has been found that the need exists for development and use of volatile blowing agent compounds which are less stable and have a shorter half-life in the atmosphere, thereby having the potential for less environmental damage.
This invention accomplishes such an improvement by producing a dimensionally stable ethylenic polymer foam, having adequate plank size, using a blowing agent that is environmentally safe.